This invention relates in general to drum support apparatus and more specifically to a drum supporting end cap, a drum assembly containing the end cap and method for fabricating the drum assembly.
Electrostatographic imaging drums are well known in the art. These drums comprise a hollow cylindrical substrate and at least one electrostatographic coating. These drums are usually supported by a hub held in place at the end of each drum by a flange extending from the hub into the interior of the drum and retained in place by an interference fit and/or an adhesive. An axle shaft through a hole in the center of each hub supports the hub and drum assembly. Where a hollow cylindrical imaging drum has a cross section circumscribed by the interior drum surface which is not fully concentric with the cross section circumscribed by the exterior drum surface, misalignment occurs between the center holes of the drum supporting hubs and the true axis of the drum. Since some electrostatographic imaging drums have inner and outer surfaces that are not perfectly concentric with each other, particularly extruded drums, these drums tend to wobble when rotated around their axle shaft. Such wobble is undesirable where precise spacing is required between the electrostatographic coating and various subsystems of the imaging system such as developing, cleaning and charging substations. In order to achieve uniform spacing between the imaging apparatus subsystems and the imaging surface of the drum, one may utilize spacing or support elements extending from the subsystems such as struts, shoes, or wheels which actually ride on the drum surface. Thus, any wobble of the drum will merely cause the subsystem to maintain its position relative to the adjacent surface of the drum as the drum wobbles as it rotates. This arrangement also compensates for variations in conicity of the drum. The use of spacing or support elements extending from electrostatographic subsystems to ride directly on a drum surface is well known in the art and is disclosed, for example, in U.S. Pat. Nos. 3,869,203 and 3,998,184. The disclosures of these patents are incorporated herein in their entirety.
One common technique for fabricating these electrostatographic imaging drums involves dip coating the drum into a coating bath to form a dielectric layer for electrographic imaging members or to form at least one electrophotographic layer for electrophotographic imaging members. During the dip coating process, one end of the drum substrate is vertically submerged with the axis of the drum substrate maintained in a vertical orientation until most or all of the external drum substrate surface is coated with the coating solution. Since the upper boundary of the applied coating layer tends to have a non-uniform thickness, the electrical properties thereat are also non-uniform. This problem is minimized by bringing the coating to the top of the substrate during dip coating so that the region actually utilized for electrostatic imaging is spaced away from the coating boundary. Since at least the bottom end of the substrate is coated and, in the latter situation, the upper end is also coated, difficulties are encountered when subsystems employed in cooperation with the electrostatographic imaging drum are spaced from the imaging surface of the electrostatic imaging drum by spacing means that ride on one or both ends of the drum to achieve more accurate spacing tolerances. These spacing means attempt to achieve spacing tolerances by compensating for situations where the drum is not perfectly round and/or the supporting drum axle shaft is not perfectly centered along the center line of the drum axis.
The spacing means which ride on the surface of the drum usually comprise a strut fitted with a roller or low friction foot. Such spacing means are well known in the art and are described, for example, in U.S. Pat. Nos. 3,869,203 and 3,998,184, referred to above. As the drum is rotated through many imaging cycles, the spacing means causes erosion of the coating on the drum substrate in the regions contacted by the spacing means and the resulting eroded coating debris forms an undesirable dust which settles on critical surfaces within the machine such as the imaging surface, corona wires, optic system components, and the like. To avoid this problem, the coating on at least one end of the drum (if the other end remains uncoated) is removed prior to installation of the drum in an electrostatographic imaging machine such as a copier, duplicator, printer or the like. Attempts to remove the coating with the aid of solvents can cause solvent splashing or fumes which can damage areas of the coating which are to be subsequently utilized for imaging. Also, removal with solvents is usually employed in combination with a wiping means such as a brush or pad which are unreliable and often fail to remove all of the coating material at the end of the imaging drum. Moreover, disposal or recovery of the solvent can be hazardous, time consuming and expensive. The coating can also be removed by use of a laser beam, but such laser systems are also complex and expensive. Moreover, the solvent/wiping or laser systems for removal of coatings are bulky and prevent achievement of high substrate population density for efficient coating and cleaning operations. Masking techniques may also be employed to remove deposited coatings from the ends of drum substrates. More specifically, a masking tape may be applied to the end or ends of a drum substrate to allow the coating to deposit on the tape instead of on the underlying substrate. The coating deposited on the tape is removed when the tape is stripped from the substrate after the coating operation. These operations all adversely affect production costs, factory floor space requirements and the like. After the coating material has been removed from at least one end of a coated drum, including any coating material deposited on the interior of at least the bottom of the drum, a hub is installed to support the drum on an axle shaft for rotation in an electrostatographic imaging machine.
When a bare strip is formed on the drum by removal of coatings adjacent to the end of the electrostatographic imaging member to expose the underlying substrate, cycling of such an imaging member during imaging in systems involving steps such as liquid development and wiping to clean the imaging surface tends to expose the edges of the electrostatographic coating, particularly any underlying more vulnerable layers, to solvent used in the liquid ink developer thereby causing deterioration of the layers and degradation of the imaging capabilities of the electrostatographic imaging member.
Hubs are often glued into place on the ends of drums. The gluing operation tends to be unreliable and expensive because a uniform bead of glue must be applied to the hub prior to insertion of the hub into one end of the drum. To ensure that the applied bead is uniform, an expensive electronic vision system is usually necessary for automated fabrication systems. Since the glue is often a thermosetting glue such as a two part epoxy resin, the hub must be hammered off when spent drum assembly components are to be recycled. Hammering can damage the hub and/or drum substrate thereby rendering these components useless for simple, direct recycling. Also, any glue and/or coating material remaining on the inside surface of the drum substrate can interfere with a subsequent electrical grounding function.
Hubs have also been used which have a peripheral support surface to space subsystems from the imaging surface of the electrostatic imaging drum by allowing a subsystem spacing means to ride on the peripheral support surface of the hub. The hub is held in place by press fitting a flange extending from the hub into the interior of the drum. This type of hub is described in Japanese Patent Publication 58-87579, published May 5, 1983. However, for hollow cylindrical substrates having a circular cross section circumscribed by an interior surface of the substrate which is not fully concentric with a circular cross section circumscribed by the exterior surface of the substrate, as is often the case with extruded substrates, the cross section circumscribed by the peripheral support surface of a hub (support ring) such as that of Japanese Patent Publication 58-87579 will be concentric with the circular cross section circumscribed by the interior surface of the substrate, and therefore, will not be concentric with a circular cross section circumscribed by the outer surface of the substrate. Thus, these types of spacing means do not achieve high spacing tolerances because they fail to compensate for situations where the interior surface of the drum is not perfectly round and/or is not concentric with the outer drum surface and/or the supporting drum axle shaft is not perfectly centered along the true center line of the drum axis. Therefore, for these situations, a peripheral support surface of the hub may be concentric with the interior surface of a drum, but will not be concentric with the exterior surface of the drum thereby causing unacceptable variations in spacing between a subsystem component and the exterior surface of the drum in precision imaging machines that demand close tolerances. An extruded cylindrical substrate having an interior surface that is not concentric with the exterior surface can be modified by mounting a raw, as extruded, substrate on a lathe using internal diameter of the substrate as the lathing datum and lathing material from the substrate in typically two or more cuts until the outer surface is concentric with the internal surface of the substrate. However, such lathing subjects the walls of the substrate to considerable force and the substrate walls must be relatively thick in order to survive the rigors of lathing. Thicker substrate walls add undesirably to the mass of the substrate and also to its cost.
Thus, there is a continuing need for improved photoreceptors that are more reliable and simpler to fabricate.